A sentence: It was dictated by the expansion of the empire, which had reached a scale in which human displacement had indeed become irreversible.

When we use SCALE, we must use ON

eg They entertain on a large scale.eg Here was corruption on a grand scale.eg On a global scale, 77% of energy is created from fossil fuels.eg We have to evaluate performance on a scale from 1 to 10.eg How much does it read on the scale?